Since polypropylene has excellent properties such as moldability, toughness, water resistance, gasoline resistance, and chemical resistance, has a low specific gravity, and is cheap, it has hithereto been widely used as various moldings, films, sheets, etc.
However, polypropylene has difficulties or points to be improved in heat resistance, rigidity, impact resistance, scratch resistance, coating properties, adhesion, printability, etc., and they become an obstacle for developing new uses.
Among them, the points to be improved in the coating properties, adhesion, printability, etc., the method for graft modification of a part or the whole of polypropylene with an unsaturated carboxylic acid or an anhydride thereof such as maleic anhydride is described in, e.g., JP-B-58-47418 (the term "JP-B" as used herein means an "examined Japanese patent publication") and JP-A-58-49736 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, even in the case of using such modified polypropylene, the impact resistance, heat resistance, rigidity, and other physical properties are not essentially improved.
Also, for improving the compatibility between polypropylene and an adhesive and/or a coating material, a medium which is a so-called primer composed of chlorinated polypropylene and toluene known as a typical composition is usually used as an intermediate layer between them.
However, in the case of coating a primer, a surface treatment such as washing with a vapor of a halogenated hydrocarbon solvent such as 1,1,1-trichloroethane is usually required before coating. However, the use of 1,1,1-trichloroethane is being regulated by an ozone problem which is one of the recent environmental pollutions.
In general, in the case of coating with a coating material after coating a polypropylene resin with a primer, sufficient adhesion is not obtained when no surface treatment with a halogenated hydrocarbon is applied.
Also, a method for improving the coating properties of a polypropylene resin molding by irradiating the surface of the molding with an ultraviolet light for improving the surface thereof is proposed in JP-A-59-147027.
However, such a method by the irradiation with an ultraviolet light, sufficient adhesion is not obtained in the case of coating the molding with a coating material.